The invention relates to a measuring head for a photoelectric measuring device, in particular a spectral reflectance measuring instrument, which includes, a source of light and an optical illuminating means which illuminates a measuring area located at a predetermined distance from the measuring head on a measuring plane at a given large angle of incidence. The device also includes an optical measuring means, the objective lens of which captures the measuring light reflected by a measuring surface in a small measuring angle range with respect to the perpendicular of the measuring surface and is connected by means of a beam waveguide with a photoelectric transformer layout, in particular a spectrometer.
A measuring head for determining optical reflectance properties which includes optical projection means to illuminate the measuring surface and optical collector means comprising an annular mirror for receiving the light reflected from the measuring surface at a predetermined angle relative to the optical axis of the optical projection or illuminating means, is known from DE 26 00 604.
It is also state of the art to evaluate the color measuring fields provided on a printed sheet, in order to improve the inking controls of an offset printing machine, not merely densitometrically, but also colorimetrically by spectral measurements. In the process, spectral reflections are used for color matching and color coordinates are calculated from them. The color coordinates are compared with corresponding set color coordinates to form resulting color deviation signals for the regulation of inking controls. To avoid measuring errors and thus errors in the control of the inking process, it is necessary to determine the color location for the co-printed color measuring fields with a high degree of accuracy over the entire wave length range. While in the case of densitometers illumination variations of the order of 2.5% due to distance variations between the illuminating means and the measuring surface may still be tolerable, such measuring errors in spectral analysis lead to undesirable errors in measuring. According to EP-A-209 860, attempts are being made to eliminate the problem by locating the source of light at the focal point of a condenser and by ensuring that the measuring area is smaller than the core area within the surface illuminated on the measuring plane. However, a relatively large amount of light is wasted in the process and the accuracy of this known apparatus is still capable of being improved.